1. Field of the Invention
The invention relates to an improved imprinting felt for use in the production of paper. The imprinting felt of the present invention contains a low level of sheet side batting and is treated with a polymer. Sheet side refers to the side of the felt which contacts the wet paper web during manufacture.
The invention further relates to an improved papermaking process using the imprinting felt. The imprinting felt of the present invention simultaneously pattern presses and dewaters the paper web.
The invention also relates to an improved paper product produced using the improved papermaking process. The paper produced according to the present invention has increased paper bulk and absorbency without having reduced strength.
2. Background of the Invention
Papermaking processes for manufacturing paper webs for use as, or in the production of tissue, towel, and sanitary paper products require the removal of water from the paper web. There are two major types of machines used for the production of these products. One type is the conventional wet press machine which is generally represented by a wet fibrous web being deposited on a Fourdrinier wire, drained with or without the aid of vacuum, transferred to a press felt and pressed onto a cylindrical drying surface. After drying, the web is creped from the drying surface and processed through a series of converting steps which may include embossing, application of glue, and lamination to form a multilayer product.
The felt used in conventional wet pressing is composed of a woven base fabric covered with batting. The base fabric provides a support for the batting and allows stable running of the felt on the paper machine. The batting material is normally a fine cut nylon filament that is needle punched onto the base fabric. The batting provides water holding capacity, forms fine capillaries that reduce the amount of rewet as the wet web exits the pressure nip and protects the base fabric from excessive machine wear.
It is important in conventional wet pressing operations, that the wet web be uniformly pressed onto the surface of the cylindrical drying surface, hereinafter referred to as a Yankee dryer. The uniform pressing of the wet web has both beneficial and detrimental effects on the drying process and paper structure. Uniform pressing reduces the amount of water that needs to be evaporated during drying of the paper web. It increases the drying rate and consolidation of the web structure, thus increasing the paper strength, but reducing the bulk and absorbency of the dried paper.
The other major type of papermaking machine for the production of absorbent and bulky paper is represented by the through-air-drying machines, one representation of which is described in U.S. Pat. No. 3,301,746 to Sanford et al., which is incorporated by reference in its entirety herein. In the process disclosed in Sanford et al., the wet paper web is pressed onto the imprinting fabric. An imprinting felt is a fabric that imprints a knuckle type pattern onto the paper web. For the purposes of the present invention, felt is understood to include a press fabric both with and without batting. After the web is placed onto an imprinting felt, it is pre-dried in an air-through-dryer. The partially dried paper web is pressed by the imprinting fabric onto the surface of the cylindrical dryer/yanker without disturbing the imprinted knuckle pattern. By contrast to the conventional wet pressing process, which uses an overall pressing, the web in Sanford et al. is pressed with the fabric knuckle pattern. While water removal and drying rates are reduced due to the non-uniform pressing, the absorbency and bulk of the paper are increased.
While the through-air-drying process of Sanford et al. increases the bulk, absorbency and softness of the paper produced, it has the drawbacks of being more complex, less efficient than conventional drying processes, and not easily implemented with existing papermaking machines.
Conventional wet pressing and through-air-drying may be considered the two extremes for the production of towel, tissue, and sanitary paper products. Others have proposed processes that represent middle grounds of these two extremes. One such process is disclosed in U.S. Pat. No. 3,537,954 to Justus, which is incorporated by reference in its entirety herein. Justus describes two methods for imprinting a knuckle pattern on a wet fiber web and depositing the web on the surface of a dryer cylinder. The first method requires using a secondary fabric to imprint the knuckle pattern onto the web after it has been uniformly pressed on the dryer surface with a conventional felt. The second method employs an imprinting fabric containing monofilament filler (batting) between the imprinting fabric strands to increase the uniformity of contact with the dryer surface.
The methods of Justus are directed to solving the problems associated with uniformity in pressing the wet web onto the dryer surface. The methods of Justus suffer from the drawback that since the imprinting fabric is not uniformly covered with a batting, water is not effectively removed from the wet web as it is pressed on the dryer surface. Because of the lack of batting, less water can be removed from the wet web during pressing and more water reenters the web as it exits the press nip.
To solve the problems inherent in Justus and to improve water removal with an imprinting fabric, U.S. Pat. No. 4,533,437 to Curran et al. discloses a method whereby the imprinting fabric was covered with batting levels greater than 153 g/m.sup.2. While batting less than 162 g/m.sup.2 does provide greater increases in bulk and absorbency as disclosed in Curran et al., Curran et al. does recognize that the batting level could not be reduced significantly below 162 g/m.sup.2 and still adequately dewater the paper. Batting levels between 152 and 162 g/m.sup.2 appear to increase absorbency and bulk, but do not provide acceptable dewatering. In addition to causing low productivity, fabrics with low levels of batting (for example, 150 g/m.sup.2) are difficult to run on a paper machine because of pulp entangling with loose batting.
Alternative solutions to the dewatering problem have taken the form of modifying the fabric or batting. U.S. Pat. No. 3,617,442 to Hurschaman discloses that conventional batting may be replaced by a synthetic, open-celled, flexible foam, such as polyurethane. The use of foam was disclosed to provide ease of manufacture of the fabric and the extension of fabric life. In another alternative, U.S. Pat. No. 4,571,359 to Dutt discloses that the base fabric could be covered with relatively large polymeric resin particles fused together to form a porous covering. The disclosed particles are from 0.15 mm to 5.0 mm in diameter. The particles were disclosed to be fused together and to the base fabric forming a covering thereover.
The present invention overcomes the disadvantages associated with the prior art. According to the present invention, the papermaking process can be carried with low levels of batting on the imprinting felt, thereby improving the bulk and absorbency of the paper product while maintaining a sufficiently high level of dewatering of the wet paper web.